Mix the saltwater at least 24 (48 is better) hours before putting the rock in. Water that hasn't thoroughly mixed will add to your die-off. (I made that mistake once..)

I would add the sand immediately after the rock. Putting the rock in first makes sure it is touching the bottom of the tank and prevents things from tipping over later if you get some sand-stirring critters in there. Once it is setup and you can see well enough, then go ahead and add the sand. It will be cloudy for awhile, but it will settle down eventually.

I think I read somewhere that mixing some saltwater in a bucket and then placing the rock into the bucket to get all the critters into the water and you can choose what critters you want to keep or dispose of then place that rock into the tank.

technically you can add the sand before or after the LR. The reason I tend to go with after, is so your LR is sitting firming on the bottom glass. If it were resting on sand, the sand could shift from currents or burrowing creatures, potentially making the rock shift which could make other rocks fall.

__________________Former advisor and planted tank geek...life's moved on though.